The fabrication of nano-scale tips is an important issue to permit maximum information to be obtained from any of the various scanned probe microscopes. Nanotips are required for well defined study of point contacts to metals or semiconductors as well as for the examination of molecules and small particles. Ultra-fine tips are demanded for future multiprobe experiments where limits on probe-to-probe spacing are a direct function of tip shape. In atomic force microscopy (AFM), where long-ranged interactions are manifest, resolution is determined not only by the last atom(s) of a tip, but by the micro-scale shape of the tip apex. Nanotips are also needed as electron field emitters in transmission electron microscopes and scanning electron microscopes or for field emission display applications. In the latter applications key issues are coherence, brightness and stability, all features that are improved by the use of nanotips. Nanotips made of magnetic materials may have uses in data storage applications.
In all of these cases, well defined, easily formed, clean and ultra-sharp tips would be advantageous. Several techniques have been developed to fabricate nano-tips: the deposition technique, Fink, H.-W.; IBM J. Pres. Develp. 1986, 30, 460-465; the build up technique, Binh V. T. Surf. Sci. 1988, 202, L539-L549, Tomitori, M.; Sugata, K.; Okuyama, G.; Kimata H. Surf. Sci. 1996, 355, 21-30; the pseudo-stationary profile technique, Binh V. T. Surf. Sci. 1988, 202, L539-L549; the field-surface-melting technique, Binh, V. T.; Garcia, N. Ultramicroscopy 1992, 42-44, 80-90; and the field-enhanced diffusion-growth technique, Nagaoka, K.; Fujii, H.; Matsuda, K.; Komaki, M.; Murata, Y.; Oshima, C.; Sakurai, T. Appl. Surf. Sci. 2001, 182, 12-19.
The first technique is based on depositing an evaporated W atom on a trimer of W(111) plane which was previously prepared by the controlled field evaporation of the apex, whereas the last four methods involve heat treatment and/or diffusion of some atoms on the apex. In all of the above techniques a W<111>tip was used, except in the field-surface-melting technique where a non-oriented Au tip was used as well.
The adsorption of molecular nitrogen on tungsten surfaces has been thoroughly investigated: Tamura, T.; Hamamura, T. Surf. Sci. 1980, 95, L293-L295; Yates, J. T.; Klein, Jr. R.; Madey, T. E. Surf. Sci. 1976, 58, 469-478; Serrano, M.; Darling G. R. Surf. Sci. 2003, 532-535, 206-212; Ehrlich, G.; Hudda, F. G. J. Chem. Phys. 1962, 36, 3233-3247; Ota K.; Usami, S. Surf. Sci. 1993, 278/288, 99-103; Müller E. W.; Tsong T. T.; Field ion microscopy; Principles and Applications, American Elsevier Publishing Company, Inc. New York 1969; and Rendulic K. D.; Knor Z., Surf. Sci. 1967, 7, 205-214. It has been found that several adsorption states are formed. Among these is the “strong-bond” state. This state arises from the dissociation of N2 on the tungsten surface followed by diffusion into the top layer of the tungsten. This causes a protrusion of W atoms, which results in a weak surface structure and therefore a decrease in the work function. Early field ion microscopy (FIM) studies of nitrogen gas on tungsten tips found that the nitrogen reaction only occurs in low field regions, where it can penetrate the ionizing barrier. Renduic et al also showed that when a W tip was exposed to nitrogen gas, holes developed on the (111) and (001) planes, resulting from the removal of W atoms. This corrosive reaction of nitrogen was explained as follows: the protrusion of W atom from the metal surface, caused by the adsorption of N2, leads to an enhanced electric field, which becomes adequate to ionize and then evaporate the protruding W atoms. This process is depicted in FIG. 1a. 
In the previous FIM work, in order to allow the interaction of nitrogen gas to take place over the entire surface of the tip, a limited dose of gas was introduced after the electric field was lowered below the initial imaging value or before any field was applied on the tip.